Process for the purification of sand



United States Patent F PROCESS FOR THE PURIFICATION OF SAND Wallace P. Behnke and Arthur J. Herrman, Wilmington,

Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Dravw'ng. Filed Nov. 25, 1957, Ser. No. 698,348 2 Claims. (Cl. 23-482) This invention relates to the production of filaments and the like from synthetic materials and, more particularly, to the melt extrusion of filaments from synthetic polymers and the spinneret assembly used therefor.

In the extrusion of molten organic filament forming compositions through spinneret orifices to form filaments, it is necessary that the molten composition be homogeneous and free from lumps, gel particles, and foreign materials which would obstruct its passage through these orifices and thereby cause irregularity in diameter of the filaments or even cessation of flow. Further, it is important that the flow of material into the spinneret orifices be uniform and, particularly, that multiple spinneret orifices in parallel be equally supplied, as inequality of supply results in variation in diameter of filaments.

In the production of filaments from a molten composition the molten material is fed into the filamentforming equipment; i.e. the spinneret, under suitable pressure and at suitable temperature from a supply duct communicating with a reservoir of molten material. For the purpose of preventing incompletely fused polymer, or foreign matter, from reaching the spinneret orifices, a spinneret pack is interposed between the supply duct and the spinneret orifices.

A spinneret pack may be composed of a large number of metal screens but such arrangements are cumbersome, the initial cost is high, and if not constructed with extreme care, operating difiiculties are encountered such as dripping from spinneret orifices, irregularity in the diameter of the filament being extruded, and the like. In addition to such objections, the difiiculty and cost of cleaning the metal screens when they have become objectionably obstructed through one cause or another, which is not infrequent even when the operation is running smoothly, is excessive.

To encounter those objections it has been suggested to replace the multiple metal screen with a sand pack such as described in U.S. 2,266,363, issued December 16, 1941, to Graves. Sand used in such packs is marketed to fit various specifications as to the size of the individual sand grain; e.g., grade A sand contains about 0.5% sand retained on a 20 mesh screen, about 95% sand retained on a 35 mesh screen, about 3.3% retained by a 42 mesh screen, and about 0.5 sand which passes the latter screen. However, even from such a specified sand fine particles (smaller than would be retained by a 65 mesh screen, or less than about mils in diameter) occasionally slip out of the sand pack and plug up the spinneret orifice or orifices, interrupting the smooth flow of the melt through the spinneret thereby causing breaks in the particular monofilament or an uneven denier filament.

It has now been found that these fine particles, which are finer than the specified minimum size for the sand grain, are originally present in the sand as agglomerates, meaning that a multitude of these tiny particles are held together by some sort of adhesion. The size of the ag- 2,984,546 Patented May 16, 1961 glomerate corresponds to the specified sand grain size range.

Commercial sand pack sand has been found to contain two types of agglomerates: one of these agglomerates appears reddish brown and these are in excess of the other type, the grayish white agglomerates. Slight pressure on these agglomerates causes them to break into many tiny particles, having a diameter of less than 10 mils, unsuitable for use in the spinneret pack. The pressure used in melt spinning a fiber-forming composition is sufiicient to break up these agglomerates, freeing the small particles and allowing them to work their way through the screens and plug the spinneret holes. Some of the particles broken out of these agglomerates are fine enough to slip through a particular orifice, the continuity of the filament at this point is either interrupted or a weak spot in the filament results. In the drawing step, following the extrusion, which is ordinarily carried out to produce optimum physical properties in. the filament, these weak spots tend to break.

Furthermore, these small sand grains are apt to damage the spinneret hole by scratching the polished surface or by breaking off particles of the spinneret material. These particles together with the sand grain causing this damage, will be found embedded in the melt-extruded film or filament which, of course, is objectionable with respect to many properties, e.g., purity, cross-section uniformity, longitudinal and transverse properties, dyeability, etc. The damaged spinneret in turn is rendered useless and replacement costs are very high.

It is, theefore, an object of the present invention to provide an improved sand for use in a filter pack assembly for the melt extrusion of filaments. A further object is to provide an economical and practical means of suitably filtering a molten organic filament-forming composition. A still further object is to provide an inexpensive spinneret assembly which will satisfactorily filter a molten organic filament-forming composition and permit its being equally supplied to multiple spinneret orifices in parallel. A further and particular object is to provide a spinneret sand pack which will supply multiple spinneret orifices in parallel homogeneously and allow extrusion of molten filament-forming compositions through smaller orifices than usual. Other objects will be apparent from the description of the invention given hereinafter.

These objects are accomplished by treating the commonly used and commercially avialable filter pack sand to break up the agglomerated fine particles. More particularly, these objects are accomplished by cleaning the sand chemically to dissolve the material binding particles of the sand in the form of objectionable agglomerates, or alternately, to subject the sand containing the agglomerates to a mechanical treatment, both treatments result ing in the same substantially agglomerate-free sand containing the fine particles. These particles, having an individual particle size of less than about 10 mils (capable of passing through a 65 mesh screen) are removed to provide a greatly improved filter pack material. In one of the embodiments of the present invention, the sand containing the agglomerates is treated by a mild sand blasting technique. A sample of commercial grade A sand is passed through an air jet at a velocity of at least about 280 feet per second bombarding the bottom of a stainless steel beaker. The impact is suificient to crush all of the agglomerates found in the sample without crushing a substantial amount of the coarse sand particles. Velocities in excess of about 380 feet per second begin to break up the pure sand particles as well. Subsequent separation then yields a sand free of agglomerates and fines. It is possible, on a large scale, to blast the filter sand and couple it to an air classifier. This is 3 econemieau'y attraenve for large quantities of sand re: quired on a commercial scale.

In the other embodiment of the present invention, the binder in each of the agglomerates is dissolved by a chemical treatment. By destroying this binder, the ag glomer'ates break up and may be easily removed by screening or elutriation. The reddish-brown agglomerates break up in hydrochloric acid of about to about 37% concentration, but this treatment does not effect the opaque, white agglomerates. The white agglomerates are found to be broken up by a caustic solution; they are unaffected by nitric acid, sulfuric acid, or mixtures of hydrochloric and nitric acids. For sand containing opaque white agglomerates, a sodium hydroxide treatment follows or precedes a hydrochloric acid treatment, followed then by washing and screening or elutriating to effectively clean and grade the sand.

The present invention will be better understood by reference to the following examples, which are given for illustration only and are not intended to be limitative. All the processes are carried out at room temperature (unless otherwise stated) and all parts and percentages are by weight. Example I In attempting to melt spin a poly(ethylene terephthalate) sample using a multiple orifice spinneret with 0.007 inch diameter holes, extreme difficulties occur in obtaining uniform continuous filaments. The sand pack used in the filter pack for this spin is filled with A grade sand obtained from the American Graded Sand Company. Investigation shows that the A sand contains numerous reddish-brown agglomerates of finer sand particles. Slight pressure on these agglomerates as exemplified by crushing them between two fingers causes them to break into many tiny particles. Elutriation of the sand is not sufficient to remove the agglomerates.

This grade A sand is successfully cleaned by soaking it overnight in a 35% hydrochloric acid solution prior to elutriation. A clean sand of uniform grade and free of reddish-brown agglomerates is obtained. The sand treated in this manner is used to fill a filter pack in a multiple orifice spinneret with 0.007 inch diameter holes and continuous filaments having much improved uniformity of the yarn cross-section are spun.

Example 11 Grade A sand (in the amount of 500 cc.) containing both white and reddish-brown agglomerates is placed in a beaker. A solution of 10% sodium hydroxide in water is poured into the beaker to cover the sand, bringing the surface level of this caustic solution about a half inch above the top surface of the sand. The sand is left untouched for 24 hours at room temperature, after which time it is washed free of the alkali, drained, and covered in the manner described above with a 35% aqueous hydrochloric acid solution. After leaving this mixture overnight at room temperature, the sand is washed free of acid with water, followed by an elutriation in a column in which the water is pressed in from the bottom.

The sand cleaned by this method does not show any white or reddish-brown grains and has a uniform beige color. Using this sand in a filter pack for melt extrusion of poly(ethylene terephthalate) after grading it to the desired size grain, gives very satisfactory spinning results and a uniform cross-sectioned filament. In addition, this filter pack has an increased pack life as compared to a filter pack containing regular grade A sand. Furthermore, it is found that sand cleaned in this manner permits spinning through 0.005 inch diameter holes, and, in spinning nylon, a 0.003 inch diameter hole spinneret is also used successfully to give a uniform cross-sectioned filament.

- Example III :Sirnilari results as in Example II above are obtained by cleaning the commercial filter pack sand by mechanical means. Grade A" sand is blasted through a yarn air jet, such as shown in US. 2,783,609, which is equipped with a 3 inch tail pipe and 8 inches of stainless steel tubing through which the sand/ air mixture is blighted under a pressure of 75 p.s.i. against a stainless steel plate which is suspended in a metal bucket about 20 inches below the air jet. The pressure with which the sandgrains are jetted against this steel plate is sufiicient to break up the fine agglomerates. The bucket serves only the purpose of collecting the blasted sand.

Close examination of the sand collected in the beaker shows that it is equal in quality to the sand cleaned according to Example II. After elutriation and grading, the sand is also successfully used in filter packs for the melt-extrusion of polyester and polyamide filaments through 0.007 and 0.005 inch diameter holes.

In repetition of this example the sand is blasted through a similar yarn jet supplied with about p.s.i. air pressure. The sand hits a rigid stainless steel plate located approximately 2 feet from the air jet, and again the sand is collected in a bucket. Under these operating conditions all of the agglomerates and approximately 15% of the A sand particles are broken. Use of this agglomerate-free sand after it is graded yields improved spinning operability also.

Example IV The sand cleaned in the two-step process of Example II is subsequently graded and used in a filter pack in an experimental melt spin of poly(ethylene terephthalate). Using a 0.007 inch diameter hole spinneret, a uniform cross-sectioned filament is extruded and spinning is maintained for about 22 hours. In a melt spin conducted parallel to this experiment but with commercial grade A sand, the quality of the spun filament degrades after approximately 6 hours and a much less uniform product results.

Using an ordinary filter pack with commercial grade A sand, it is not possible to melt spin poly(ethylene terephthalate) through 0.005 inch diameter holes to produce a yarn of high quality denier. However, spinning of poly(ethylene terephthalate) filaments using a filter pack comprising sand cleaned by either the two-step chemical treatment of Example 11 or the mechanical treatment of Example III is successful, and a yarn of 0.25 denier per filament is produced.

Example V Six experimental spinning packs are prepared for a spinneret assembly, three from the treated, agglomerate free, A sand and three from as-received A sand. The spinneret contains 30 holes of 0.005 inch diameter each. In the spinning of poly(ethylene terephthalate) through this assembly, each is spun for 1 hour after which filament-to-filament uniformity is determined.

These data show clearly that in melt spinning from small diameter holes broken filaments can be avoided by using a filter pack with agglomerate-free sand. Furthermore, the erratic filament-to-filament uniformity observed when using untreated sand in the packs indicates so-called slow holes, probably due to sand particles partially plugging the holes, and yielding weaker filaments which would break in the drawing step ordinarily carried out subsequent to the spinning of commercial fibers to induce optimum physical and aesthetic properties.

Thus, either the chemical or the mechanical treatment for the cleaning of commercial filter pack sand, accord' ing to the present invention, yields a uniform, agglomerate-free sand. Since with the treatment described in detail above the sand will have a much extended filter pack life, the low cost of either of the processes is highly justified by the better yarn qualities obtained with them, the much improved spinning continuity, and the larger filter pack life. The agglomerate-free sand will not damage or obstruct the spinneret hole, nor will fines slip through the spinneret hole and cause lower purity of the resulting melt extruded filament. The sand can be used much longer in filter packs without the necessity of being replaced or discarded, and it permits the use of smaller diameter spinneret holes for the spinning of finer denier filaments than would be possible with regular commercial sand.

Optimum conditions for removal of the reddishbrown agglomerates from grade A sand include a treatment of about 30 minutes at about 95 C. with about 35% aqueous hydrochloric acid.

Concentrated hydrochloric acid is suitable to dissolve the binder, as are other acids such as concentrated or dilute (about 50%) sulfuric and nitric acids, as well as mixtures of concentrated nitric acid and hydrochloric acid.

Optimum conditions for removal of opaque, white agglomerates from this sand includes a treatment with at least about aqueous sodium hydroxide solution at 95 C. over a period of about 8 hours.

Obviously, there is no upper time limit for any of the above treatments, since neither a 10% caustic solution nor a 35% hydrochloric acid solution dissolve any of the essentially pure silicate grains of the grade A sand.

To destroy the white opaque agglomerates, the sand is preferably treated with a 10% sodium hydroxide solution but other water soluble strong alkalies such as potassium hydroxide will accomplish the same result. If conducted at room temperature, it is desirable to continue the treatment for about 12 hours or more, whereas at about 95 C., a minimum time of 8 hours seems to be necessary. The hydrochloric acid treatment is desirably continued for about 12 hours at room temperature or for at least 30 minutes at about 95 C. It will be understood that other acids of equivalent strength also will destroy the reddish brown agglomerates.

In the above specification, the term destroying agglomerates does not mean that the agglomerates are dissolved in the chemical treatment, but the binder between the fines in these agglomerates dissolves and the fines separate. Since these fines are of a smaller diameter than the recommended size for sand grains usable in filter packs, an elutriation and grading step is conducted after this chemical treatment. The same, of course,

holds true for the sand blasting technique, in which fines originate from the agglomerates and also from the occasional breakage of a regular sand grain. Obviously, any device may be used which produces sufiicient velocity for blasting the sand against any suitably hard surface, such as a steel plate to accomplish the same result. In other words, the form or shape of the fluid sand blast ing jet is unimportant.

I claim:

1. In a process for preparing a clean filter sand having a substantially uniform sand grain size, for spinneret pack assemblies for molten organic filament-forming polymeric materials, from a commercial sand comprising substantially uniform single sand grains of given size and agglomerated bodies of said given size consisting of fine sand particles of a size smaller than said given size, said particles bonded together by an adherent binder material; the improvement consisting of the steps of first mechanically fracturing said binder material of the agglomerated bodies by impacting said bodies against a hard surface at a velocity from about 280 to about 380 feet per second while maintaining the single sand grain size unchanged to liberate said fine sand particles upon breakup of the agglomerated bodies, and then removing said fine sand particles and binder material to leave only said single sand grains of said substantially uniform given grain me.

2. In a process for preparing a clean non-clogging filter sand having a substantially uniform sand grain size, for spinneret pack assemblies for molten organic filament-forming polymeric materials, from a commercial sand comprising substantially uniform single sand grains of given size and agglomerated bodies of said given size consisting of fine sand particles of less than said given size, said particles bonded together by an adherent binder composition; the improvement consisting of the steps of first dissolving the binder composition, while maintaining the single sand grain size unchanged to liberate said fine particles upon breakup of said bodies, by subjecting said binder composition separately to the solvent action of aqueous hydrochloric acid of from about 10% concentration to about 37% concentration for at least thirty minutes and the solvent action of an aqueous caustic solution of about 10% concentration for about eight hours, and then removing said fine sand particles and dissolved binder composition to leave only said single sand grains of substantially uniform grain size.

References Cited in the file of this patent UNITED STATES PATENTS 1,983,272 Earle Dec. 4, 1934 2,122,217 Sisson June 28, 1939 2,266,363 Graves Dec. 16, 1941 2,266,368 Hull et al. Dec. 16, 1941 2,556,005 Segrove et a1. June 5, 1951 2,813,318 Horth Nov. 19, 1957 L Attesting Officer UMTEI) STATES PATENT UFFICE CERTIFICATE OF CORRECTION Patent No; 2 984546 May 16, 1961 Wallace Po Behnke et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent. should read as corrected below.

Column 1, line 43, before "irregularity" insert nonuniform flow to the spinneret orifices column 2, line 31 for "'theefore" read therefore line 48, for "avialable" read available Signed and sealed this 10th 'day of October 1961.,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Commissioner of Patents USCOMM-DC 

1. IN A PROCESS FOR PREPARING A CLEAN FILTER SAND HAVING A SUBSTANTIALLY UNIFORM SAND GRAIN SIZE, FOR SPINNERET PACK ASSEMBLIES FOR MOLTEN ORGANIC FILAMENT-FORMING POLYMERIC MATERIALS, FROM A COMMERCIAL SAND COMPRISING SUBSTANTIALLY UNIFORM SINGLE SAND GRAINS OF GIVEN SIZE AND AGGLOMERATED BODIES OF SAID GIVEN SIZE CONSISTING OF FINE SAND PARTICLES OF A SIZE SMALLER THAN SAID GIVEN SIZE, SAID PARTICLES BONDED TOGETHER BY AN ADHERENT BINDER MATERIAL, THE IMPROVEMENT CONSISTING OF THE STEPS OF FIRST MECHANICALLY FRACTURING SAID BINDER MATERIAL OF THE AGGLOMERATED BODIES BY IMPACTING SAID BODIES AGAINST A HARD SURFACE AT A VELOCITY FROM ABOUT 280 TO ABOUT 380 FEET PER SECOND WHILE MAINTAINING THE SINGLE SAND GRAIN SIZE UNCHANGED TO LIBERATE SAID FINE SAND PARTICLES UPON BREAKUP OF THE AGGLOMERATED BODIES, AND THEN REMOVING SAID FINE SAND PARTICLES AND BINDER MATERIAL TO LEAVE ONLY SAID SINGLE SAND GRAINS OF SAID SUBSTANTIALLY UNIFORM GIVEN GRAIN SIZE.
 2. IN A PROCESS FOR PREPARING A CLEAN NON-CLOGGING FILTER SAND HAVING A SUBSTANTIALLY UNIFORM SAND GRAIN SIZE, FOR SPINNERET PACK ASSEMBLIES FOR MOLTEN ORGANIC FILAMENT-FORMING POLYMERIC MATERIALS, FROM A COMMERCIAL SAND COMPRISING SUBSTANTIALLY UNIFORM SINGLE SAND GRAINS OF GIVEN SIZE AND AGGLOMERATED BODIES OF SAID GIVEN SIZE CONSISTING OF FINE SAND PARTICLES OF LESS THAN SAID GIVEN SIZE, SAID PARTICLES BONDED TOGETHER BY AN ADHERENT BINDER COMPOSITION, THE IMPROVEMENT CONSISTING OF THE STEPS OF FIRST DISSOLVING THE BINDER COMPOSITION, WHILE MAINTAINING THE SINGLE SAND GRAIN SIZE UNCHANGED TO LIBERATE SAID FINE PARTICLES UPON BREAKUP OF SAID BODIES, BY SUBJECTING SAID BINDER COMPOSITION SEPARATELY TO THE SOLVENT ACTION OF AQUEOUS HYDROCHLORIC ACID OF FROM ABOUT 10% CONCENTRATION TO ABOUT 37% CONCENTRATION FOR AT LEAST THIRTY MINUTES AND THE SOLVENT ACTION OF AN AQUEOUS CAUSTIC SOLUTION OF ABOUT 10% CONCENTRATION FOR ABOUT EIGHT HOURS, AND THEN REMOVING SAID FINE SAND PARTICLES AND DISSOLVED BINDER COMPOSITION TO LEAVE ONLY SAID SINGLE SAND GRAINS OF SUBSTANTIALLY UNIFORM GRAIN SIZE. 